1. Field of the Invention
The present invention relates generally to optical measuring equipment and methods, and in particular to such equipment and methods for measuring birefringence in such measures as differential group delay.
2. Technical Background
Optical fiber is the favored transmission medium for long-distance telecommunication systems because of its very large bandwidth (that is, data carrying capacity), immunity to noise, and relatively low cost. Attenuation in silica optical fiber has been reduced to such low levels that it is possible to transmit data over hundreds of kilometers without the need for amplifiers or repeaters. The data carrying capacity of a fiber communication system over relatively short distances is in large part dictated by the speed of the electronics and opto-electronics used at the transmitter and receiver. At the present time, the most advanced commercially available optical receivers and transmitters are limited to about 10 gigabits/sec (Gb/s), although 40 Gb/s systems are being contemplated.
However, over the longer distances typical for telecommunications, dispersion of various types may limit the useful bandwidth. A cylindrical optical fiber of fairly large cross section can transmit a number of waveguide modes exhibiting different spatial power distributions. The propagation velocity differs between the fundamental mode and the higher-order modes in an effect called modal dispersion. An optical signal impressed by a transmitter on the fiber will typically contain a distribution of all the modes supportable by the fiber. Because of the modal dispersion, the different modes after traversing a long section of fiber will arrive at the receiver at slightly different times. The transmission rate is limited by the dispersion integrated along the transmission length.
In order to avoid modal dispersion, most modem fiber communication systems intended for long-distance transmission rely upon single-mode fiber. In the case of a simple fiber with a core and cladding, the core of a single-mode fiber is so small, taken in conjunction with the difference of refractive indices between the core and the cladding, that the fiber will support only the fundamental mode. All higher-order modes are quickly attenuated over the distances associated with long-distance telecommunication. The description is more complicated for a profiled fiber or for a fiber having multiple cladding layers, but it is well known how to fabricate and test a fiber such that it is single-moded.
A circularly symmetric single-mode fiber in fact supports two fundamental transverse modes corresponding to the two polarization states of the lowest-order modes. To a fair approximation, these two lowest-order modes are degenerate in the circular geometry of a fiber and have the same velocity of propagation so there is no polarization dependent dispersion. However, as will be explained later, polarization dependent dispersion can arise in a realistic fiber.
In the past, high bit-rate transmission over long distances of single-mode fiber has been limited by chromatic dispersion, also characterized as group velocity dispersion. A data signal impressed on an optical carrier signal causes the optical signal to have a finite bandwidth, whether it be considered produced by the spectral decomposition of a pulsed signal or by the data bandwidth of an analog signal. Generally, the velocity of propagation or propagation constant of an optical signal, is primarily dependent upon the refractive index of the core, varies with optical frequency. As a result, the different frequency components of the optical signal will arrive at the receiver at different times. Chromatic dispersion can be minimized by operating at wavelengths near zero dispersion, about 1300 nm for silica, or by other methods for compensating dispersion.
Despite its circularly symmetric design, real optical fiber is typically birefringent. This means that the two lowest-order axial modes are not degenerate, and the fiber at any point may be characterized as having a fast axis and a slow axis. The two modes traveling along the fiber with their electric field vectors aligned respectively with the fast and slow axes of the fiber will propagate relatively faster or slower. As a result, the group velocity of a signal traversing the fiber is a function of the polarization state of the optical signal. Birefringence can arise from internal or external sources. The fiber may have been drawn with a slight physical non-circularity. The fiber may be installed such that a bend, lateral load, anisotropic stress, or a twist is applied to it. The birefringent interaction is complicated by coupling of the two modes also occurring at fiber twists, bends, or other causes. The coupling causes energy to transfer between the orthogonal modes. But even with mode coupling, the group delay continues to spread out, resulting in a significant polarization mode delay or dispersion (PMD). The cause of mode coupling is not completely understood, but it is modeled by a statistical model of randomly occurring mode-coupling sites with an average distance between the sites (mode coupling length), which typically assumes a value between about 5m and 100 m. The exact mode coupling length depends on the deployment of the fiber and is not usually characteristic of the intrinsic fiber birefringence.
It is estimated that above about 10 Gb/s, polarization mode dispersion limits fiber bit rates more than other types of dispersion. Polarization mode dispersion also degrades cable television (CATV) systems by introducing composite second-order distortion and signal fading.
Some fiber manufacturers draw their fiber with a small continuous twist applied to the fiber so that manufacturing anisotropies do not allow the fast and slow modes to always be aligned to a propagation mode. Thereby, the difference in propagation delay between the two modes is lessened, resulting in reduced PMD. A further technique for reducing net PMD over a long distance is to periodically reverse the direction of the manufacturing twist.
In the past, polarization mode dispersion has been treated as a time-dependent quantity requiring a statistical description. PMD has been typically measured on long lengths (1 km or more) of fiber wound under low tension about a spool of large diameter. The bending and stress induced by higher tension winding on a smaller shipping spool affect the birefringence and mode coupling and, hence, the average PMD experienced. However, setting up such a test demands time and resources. Further, the 1 km sections of fiber cut from the shipping spool or the production line cannot be otherwise used, and the testing represents a loss 1 km of fiber, which for a standard 25 km spool is a loss of 4%.
Accordingly, it is desired to measure the effects of polarization mode dispersion expected to be experienced in a realistic environment with out the need to test long lengths of fiber. It is further desired to measure the effects of polarization mode dispersion in an accurate and deterministic fashion.
The invention includes a method and apparatus for measuring polarization mode dispersion in an optical fiber, preferably quantified as differential group delay between the two fundamental polarization modes.
In one aspect of the invention, one or more incoherent light sources are used in conjunction with optical bandpass filters to provide light to a polarimeter arranged to measure birefringence in an optical fiber. The polarimeter measures how the fiber affects the state of polarization of light passing through it, preferably by a measurement of polarization mode delay or dispersion.
Visible laser light may be switched into the fiber for visual alignment. Laser light of wavelength comparable to that of the incoherent sources may also be switched into the fiber and electronically detected to complete the alignment. An optical switch can be positioned at the output of the fiber under test to switch the light alternatively to the polarimeter and the alignment detector without affecting the measurement of polarization mode dispersion.
The fiber may be subjected to a selected amount of twist along its length. The measured twist-dependent polarization mode dispersion may be used to determine several optical properties of the fiber. The fiber may also be subjected to a selected amount of load or otherwise stressed during its testing.
The value of polarization mode dispersion measured for a short length of fiber may be empirically mapped to values for longer fiber, with the polarization mode coupling length being intermediate the two fiber lengths. The mapping may be used to measure the mode coupling length.